Validator observability PR adds submission lifecycle tracking

New PR #40 introduces detailed logging across the full submission path from arrival through reward, splitting the process into seven stages with new diagnostic fields. The update clarifies the distinction between pool acceptance and final batch selection/reward status, which only appear in sealed window records rather than immediate verdict polls.

• •Seven-stage submission path now logged: arrival → drand_check → queue_wait → proof_verify → pool → selection → rewarded
• •New fields track drand rounds, timing (ms), and rejection reasons for better debugging
• •Batch_filled status now distinguishes between seal, drain, trigger, prompt, cooldown, and ordering conditions
• •Selected_for_batch and rewarded fields only populate after window seal; not in async verdict polls